A method of driving a permanent magnet synchronous electric motor includes sensing or estimating a back electromotive force induced in at least a winding of the motor by the rotation of a rotor of the motor; and reading, from a memory, values of a first voltage waveform having a phase angle with respect to the back electromotive force. The method also includes generating a driving voltage corresponding to the sum of values of a control voltage, obtained as product of the values of the first voltage waveform by a first coefficient determined as a function of a desired value of motor torque, and values of a cancelation voltage of the back electromotive force. The method also includes applying the driving voltage at the motor winding.
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6. A method of driving an electric motor having a plurality of windings and a rotor associated therewith, comprising:
determining a back electromotive force induced in at least one winding by rotation of the rotor;
reading, from a memory, values of a first voltage waveform having a phase angle with respect to the back electromotive force;
generating a driving voltage based upon values of a control voltage, obtained from the values of the first voltage waveform and a first coefficient determined as a function of a desired value of motor torque, and values of a cancelation voltage of the back electromotive force; and
applying the driving voltage to the at least one winding.
1. A method of driving a permanent magnet synchronous electric motor having a plurality of windings and a rotor associated therewith, comprising:
sensing or estimating a back electromotive force induced in at least one winding by rotation of the rotor;
reading, from a memory, values of a first voltage waveform having a phase angle with respect to the back electromotive force;
generating a driving voltage corresponding to a sum of values of a control voltage, obtained as a product of the values of the first voltage waveform by a first coefficient determined as a function of a desired value of motor torque, and values of a cancelation voltage of the back electromotive force; and
applying the driving voltage to the at least one winding.
17. A device for driving an electric motor having a plurality of windings and a rotor associated therewith, comprising:
a circuit configured to determine a back electromotive force induced in at least one winding by rotation of the rotor;
a memory adapted to store values of a first voltage waveform;
a control circuit configured to
read, from said memory, values of the first voltage waveform having a phase angle with respect to the back electromotive force, and
generate a driving voltage based upon values of a control voltage, obtained from the values of the first voltage waveform and a first coefficient determined as a function of a desired value of motor torque, and values of a cancelation voltage of the back electromotive force; and
a power stage configured to apply the driving voltage to the at least one winding.
13. A device for driving a permanent magnet synchronous electric motor having a plurality of windings and a rotor associated therewith, comprising:
a feedback circuit configured to sense or estimate a back electromotive force induced in at least one winding by rotation of the rotor;
a memory adapted to store values of a first voltage waveform;
a control circuit configured to
read sequentially from said memory the values of the first voltage waveform having a phase angle with respect to the back electromotive force, and
determine a first coefficient as a function of a desired value of motor torque;
a first multiplying circuit configured to generate values of a control voltage obtained as a product of the values of the first voltage waveform by the first coefficient; and
a power stage coupled with said control circuit, configured to apply, at the at least one winding, a driving voltage corresponding to a sum of the control voltage and a cancelation voltage of the back electromotive force.
2. The method of
determining a phase angle (β) with respect to the back electromotive force as a function of motor speed ω, of a resistance R and inductance L of the at least one winding, such as to cause a desired phase angle between a current flowing throughout the at least one winding and the back electromotive force;
generating values of the control voltage by multiplying the values of the first voltage waveform by the first coefficient;
determining a second coefficient corresponding to a value of an amplitude of the back electromotive force;
reading, from the memory, values of a second voltage waveform in phase with the back electromotive force;
generating values of the cancelation voltage of the back electromotive force, by multiplying the second coefficient by the values of the second voltage waveform in phase with the back electromotive force; and
generating the driving voltage as the sum between the control voltage and the cancelation voltage.
4. The method of
determining a sum between the control voltage and the cancellation voltage, and determining a phase angle between the sum and the back electromotive force;
determining said first coefficient, equal to a value of an amplitude of the sum between the control voltage and the cancelation voltage; and
generating the driving voltage corresponding to a product between the values of the first voltage waveform by the first coefficient.
5. The method according to
7. The method of
8. The method of
9. The method of
determining a phase angle (β) with respect to the back electromotive force as a function of motor speed ω, of a resistance R and inductance L of the at least one winding, such as to cause a desired phase angle between a current flowing throughout the at least one winding and the back electromotive force;
generating values of the control voltage by multiplying the values of the first voltage waveform by the first coefficient;
determining a second coefficient corresponding to a value of an amplitude of the back electromotive force;
reading, from the memory, values of a second voltage waveform in phase with the back electromotive force;
generating values of the cancelation voltage of the back electromotive force, by multiplying the second coefficient by the values of the second voltage waveform in phase with the back electromotive force; and
generating the driving voltage as the sum between the control voltage and the cancelation voltage.
11. The method of
determining a sum between the control voltage and the cancellation voltage, and determining a phase angle between the sum and the back electromotive force;
determining said first coefficient, equal to a value of an amplitude of the sum between the control voltage and the cancelation voltage; and
generating the driving voltage corresponding to a product between the values of the first voltage waveform by the first coefficient.
12. The method according to
14. The device of
15. The device of
16. The device of
18. The device of
19. The device of
20. The device of
21. The device of
22. The device of
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This invention relates to techniques for driving electric motors, and, more particularly, to a method of driving and a related driver of a permanent magnet synchronous electric motor.
Electrically switched DC motors, such as stepper motors or more generally brushless motors, are used in numerous control and regulation applications and also in systems for driving mass memory devices such as hard disks, floppy disks, optical disks, CD-ROMs, etc.
Hereinafter reference will be made to a three-phase electric motor, though the same observations also hold for a generic poly-phase motor.
Brushless motors may be driven using an integrated circuit, commonly called “smooth driver”, of the type shown in
Typically, a brushless motor is driven by properly supplying the phases of the motor synchronously with the instantaneous position of the motor. This may be done by energizing sequentially two windings of the motor with positive and negative voltages, respectively, leaving a third winding in a high impedance state. When a brushless “sensorless” motor is driven, the not energized winding is exploited for sensing the position of the rotor. The driving voltages or currents applied to the motor windings, instead of having a pre-established constant level during each switching phase, have a certain digitized pre-established (non constant) driving voltage or current waveform stored in a nonvolatile static memory device, for example, in an EPROM or EEPROM memory.
This technique is well known in the art, for example from the European patents EP 800262, EP 800263, EP 809349 and from U.S. Pat. No. 6,137,253 the disclosures of which are herein incorporated by reference in their entireties, and for this reason will not be illustrated further. A basic scheme of the driver is depicted in
Profile A
Memory device storing the voltage
waveform for winding A
Profile B
Memory device storing the voltage
waveform for winding B
Profile C
Memory device storing the voltage
waveform for winding C
Power A
Half-bridge for driving winding A
Power B
Half-bridge for driving winding B
Power C
Half-bridge for driving winding C
Motor
Motor
Position
Position signal of the rotor
Controller
Control circuit
Speed Measure
Circuit for measuring the rotor
speed starting from the signal of
the rotor position
Speed
Rotor speed
Speed Control
Control circuit of the rotor speed
Torque Control
Motor torque optimization circuit
KVAL
Signal proportional to a desired
value of motor torque
Addr. Generator
Generator of memory addresses for
reading waveforms
Torque Optim
Phase angle (start of a reading
from the memory) in respect to the
BEMF of a voltage Vin applied to a
winding
Reset address
Start signal for reading a
waveform from the memory for
obtaining a synchronous waveform
with the rotor position
Profile Out
Voltage waveform read from the
memory
Vin
Control voltage
PhA current
Phase current in winding A
In order to properly supply the windings, the position and speed of the rotor of the motor are determined with a feedback circuit for sensing or estimating the back electromotive force (BEMF) induced in the tristated winding.
The driver allows a voltage mode driving typically used for permanent magnet synchronous motors PMSM because it is simpler than current mode driving. The values KVAL and Torque Optim are provided as inputs; and, as a function thereof, a phase voltage Vin to be applied at the motor windings is generated. In order to make the applied voltage independent from eventual fluctuations of the supply voltage of the motor, closed-loop compensation methods may be used as well as open-loop systems with feed-forward compensation. An example of feed-forward compensation is described in the U.S. Pat. No. 6,150,963 in the name of the same applicant, herein incorporated by reference in its entirety.
Considering purely sinusoidal or pseudo-sinusoidal (i.e. step waveforms that approximate sinusoidal waveforms) phase voltages, the driving of each winding of the electric motor is fully described by the phasors of
Torque
Phase angle (start of a reading from the
Optim
memory) in respect to the BEMF of a voltage
Vin applied to a winding
KVAL
Signal proportional to a desired value of
motor torque
BEMF
Back electromotive force induced by the
rotation of the motor
R
Resistance of the winding
L
Inductance of the winding
I
Phase current flowing throughout the winding
Vin
Control voltage of the winding
Vdiff
Resistive-inductive voltage drop on the
winding
γ
Phase angle between phase current and back
electromotive force
B
Phase angle between the resistive-inductive
voltage drop and the induced back
electromotive force
A
Phase angle between the resistive-inductive
voltage drop and the motor coil current
Motor
Motor winding
phase
Ω
Angular speed of the motor
As clearly shown by the phasors of
Typically, the value KVAL is fixed as a function of the desired motor speed, and the value Torque Optim is fixed accordingly to optimize the motor torque once the desired value of speed is established.
Being that the motor torque of a PMSM motor is a function of the phase angle between the stator phase current and the related BEMF induced at the same phase, it may be important in voltage mode systems to have the possibility of controlling the stator current i with respect to the BEMF (on its rotational function of the rotor position), besides its amplitude.
In the known voltage mode driving of
A drawback of this driving technique is that there is not solely a relationship between the phase angle between the voltage Vin and the BEMF and the phase angle between the BEMF and the phase current i. An optimal phase angle value (Torque Optim “optimal”) could be obtained, for example, through the calibration procedure or through appropriate calculation carried out by a microcontroller or through look-up table or other method. For ease it is common practice to use a constant phase angle value Torque Optim during the normal functioning of the motor: this ensures good performance in most functioning conditions, in which the motor is in steady state conditions, i.e. with a constant rotation speed and resistive torque.
As described by the phasors of
In particular,
Similarly,
In transient conditions, the PMSM motor is not driven in conditions of maximum efficiency.
Having a current flowing throughout the windings of the motor that varies in phase depending on the requested motor torque (KVAL), may be considered, in certain cases, an undesirable limitation.
Studies carried out by the applicant have led considering the variation of the phase angle of the resistive-inductive drop Vdiff with respect to the BEMF, when the value of the requested torque KVAL varies, a cause of the above mentioned drawback. Indeed, in the known voltage mode drivers, the phase of the voltage Vdiff with respect to the BEMF, being a function of the amplitude of the control voltage Vin, is non constant during a transient increase/decrease of the requested motor torque.
A method of driving has been found, implementable in a related driver of a permanent magnet synchronous electric motor, in which the phase of the current flowing throughout the windings of the motor does not depend upon the value of desired torque. This excellent result may be obtained by controlling directly the resistive-inductive drop Vdiff of a PMSM motor winding through the steps of: sensing or estimating a back electromotive force induced in at least one winding of the motor by the rotation of a rotor of the motor; reading, from a memory, values of a first voltage waveform having a phase angle with respect to the back electromotive force; and generating a driving voltage, as a function of the values read from the memory with the phase angle and of the first coefficient, corresponding to the sum between the values of a control voltage obtained as product of the values of the first voltage waveform by a first coefficient determined as a function of a desired value of motor torque, and values of a cancelation voltage of the back electromotive force. The method may also include applying the driving voltage at the motor winding.
According to an embodiment, implementable in an innovative driver of a permanent magnet synchronous electric motor, the phase voltage is generated through the following operations: determining this phase angle with respect to the back electromotive force as a function of the speed ω of the motor, of the resistance R and of the inductance L of the winding, such as to cause a desired phase angle between a current flowing throughout the winding and the back electromotive force; generating values of the control voltage by multiplying the values of the first voltage waveform by the first coefficient corresponding to the desired value of motor torque; determining a second coefficient corresponding to a value of amplitude of the back electromotive force; reading from the memory values of a second voltage waveform in phase with the back electromotive force; generating values of the cancelation voltage of the back electromotive force by multiplying the second coefficient by the values of the second voltage waveform; and generating the driving voltage as the sum between the control voltage and the cancelation voltage.
According to another embodiment, the phase voltage is generated through the following steps: determining the phase angle with respect to the back electromotive force corresponding to the phase angle with respect to the back electromotive force by determining the sum between the control voltage and the cancelation voltage; determining the first coefficient equal to a value of amplitude of the sum between the control voltage and the cancelation voltage; generating the driving voltage corresponding to the product between the values of the first voltage waveform by the first coefficient.
An innovative driver of a permanent magnet synchronous electric motor is also disclosed.
In this specification the case of a sinusoidal control voltage Vin will be considered, though this particular voltage waveform is just an example, since it is possible to use voltage waveforms as disclosed in U.S. Pat. Nos. 6,137,253 or 7,834,568 in the name of the same applicant, or of any other type adapted to generate sinusoidal or pseudo-sinusoidal (i.e. that approximate a sinusoid) currents through the motor windings.
The method of this disclosure performs a direct driving of the resistive-inductive drop Vdiff, instead of driving the voltage Vin=Vdiff+BEMF, as in the known device of
A first embodiment of the method is schematically described by the electric circuit of
The amplitude of the cancelation voltage Vin_2 of the back electromotive force BEMF may be calculated (BEMF=Kt*ω), starting from the speed ω of the motor and of its torque constant Kt, and its phase may be determined by monitoring the zero-crosses of the BEMF. As an alternative, with a circuit that generates an analog or digital replica signal of the BEMF, it is possible to determine the cancelation voltage Vin_2 using this replica signal.
The waveform Profile2_Out is then amplitude-modulated by a factor ProfileAmplAdj adapted to make Vin_2=BEMF; the waveform Profile1_Out instead is modulated in order to control the amplitude of the current i. Indeed, given that the voltage Vin_1 is directly proportional to the value KVAL of the motor torque to be provided and given that, at a constant speed, the impedance of the winding is constant, it is thus possible to control directly the current by controlling the value KVAL.
The driving technique of this disclosure is not affected by this drawback because the current i forced throughout the winding does not have the discontinuities of the quantized waveform, because the inductance of the winding acts as a low-pass filter, as shown in
The present disclosure allows improvement of the driving system for PMSMs (Permanent Magnet Synchronous Motors), typically used as spindle motors in hard disks, CD, DVD, etc. In particular, the present disclosure allows driving the PMSM motor in a voltage mode with sinusoidal (or pseudo-sinusoidal) current waveforms having a phase relation, with respect to the reference signal (that may typically though not necessarily be the BEMF), that may be directly controlled without using current sensors and without the need of realizing a closed loop control. This allows obtaining performances similar to those of a current mode driving, though using simpler and lower cost circuits and reducing or preventing stability problems associated with closed-loop control schemes.
With the present disclosure it is also possible to prevent the inversion of polarity of the current throughout the windings, as occurs in traditional voltage mode controls when the voltage applied at the windings is smaller than the BEMF value.
Galbiati, Ezio, Pascale, Paolo, Magni, Federico, Boscolo, Michele Berto
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